1. Field of the Invention
This invention relates to an apparatus for injecting molten resin into a mold cavity, and more particularly to a plug-in type nozzle with a valve gate for runnerless and sprueless type molding of thermoplastic materials.
2. Description of the Prior Art
Hitherto, in the field of injection molding of thermoplastic materials, there has been no effective injection nozzle having a gate which opens and closes distinctively. With the conventional injection nozzle, molten resin often leaks from the gate causing drooling or stringing when the gate should close. Therefore in order to obtain product of good quality, extra parts such as runner or sprue are required for providing instantaneous injection or for disposing the leaked resin. Such molded product with runner or sprue needs additional finishing process of trimming or spotfacing. It is quite difficult to bring automation to these finishing process completely and inexpensively.
There have been various attempts to eliminate sprue or runner without success. There tried two kinds of runnerless or sprueless type injection molding apparatus, so-called heat gate type and valve gate type.
In the heat gate type, the resin around the gate is solidified or melted by controlling the temperature around the gate using cooling water or electric heater etc., so that the gate closes or opens. This type of apparatus requires precise temperature control and can not prevent leakage of resin from the gate causing drooling or stringing especially for resins with low viscosity such as nylon-6.
In valve gate type, a needle opens or closes the gate mechanically. This type of apparatus requires a complicated driving power such as oil pressure mechanism outside the nozzle in order to activate said needle valve. There have been attempts to eliminate such an external driving power by adopting a valve gate which works automatically by the pressure of molten resin. However this kind of valve gate requires a strong spring for resisting the high injection pressure of 500-5000 Kg/cm.sup.2. Therefore the nozzle necessarily becomes so cumbersome that it is impossible to mold very small products precisely.
Nowadays several plug-in type injection nozzles have been developed for simplifying designing of the mold cavity. These plug-in type nozzles belong to either heat gate type or valve gate type. So plug-in type nozzles can not get rid of the above mentioned disadvantages either.
Thus, there has been no effective plug-in type, runnerless type or sprueless type injection molding nozzle, although there have been strong desires for an injection nozzle without the above mentioned disadvantages.